1. Field of the Invention
The present invention relates to a finder optical system, which is used in a digital still camera and the like, using a secondary image-formation method which reverses a primary image consisting of an inverted image to a secondary image consisting of an erecting image in order to observe via an ocular optical system, and an imaging apparatus comprising the same.
Priority is claimed on Japanese Patent Application No. 2004-367998, filed Dec. 20, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, there is an increasing demand for digital still cameras, including a digital single-lens reflex camera which uses an image pickup device whose size is a fraction of the film diagonal length in a silver salt 35 mm film camera. Since the angle of the view of the finder optical system in such a camera must be wide enough for a high-grade camera, it is necessary to shorten the focal length of the overall finder optical system. It is also necessary to provide an erecting optical system between the primary image formation face of the finder optical system and the ocular optical system. In a finder optical system, the pupil diameter and eye point position are fixed, irrespective of the focal length of the ocular optical system.
One conventional method which is generally known is a secondary image formation method using an objective optical system to form an image temporarily in the finder optical system, and then reforming the image by reversing this image using a lens array, thereby reducing the load on the ocular optical system. For example, this type of finder optical system is disclosed in Japanese Patent No. 2520683 and Japanese Patent No. 3382317.
An optical system for focal point detection in a single-lens reflex camera generally has a configuration for adjusting the focal point, wherein a luminous flux for taking image is transmitted through a main mirror for switching the optical path and is led in the opposite direction from the finder optical system by a sub-mirror (Japanese Patent No. 3010813).
Japanese Unexamined Patent Publication No. 2002-139773 proposes a finder device which uses a secondary image formation-type finder optical system to superimpose a display image within the field of view.